The leukocyte complement (C) system consists of eight types of membrane C receptors, various C components synthesized by leukocytes, and plasma C components that interact with leukocytes. The objective of this project is to characterize the function of the leukocyte C system. The specific aims are as follows: (1)\examine the in vitro immune response of lymphocytes and determine if the leukocyte C system has a role in recognition of alternative pathway (AP) activators through either factor B cleavage of membrane C5 or the binding of factor H (H) to membrane H receptors (H-R). Characterize the suppression of lymphocyte activation by CR2 (C3d-receptor) ligands and determine if this represents a mechanism for discrimination of nonactivators of the AP. (2)\Examine the role of the leukocyte C system in the tumoricidal activity of macrophages and natural killer cells. Determine whether serum-\or effector cell-derived C3 is deposited onto AP-activating tumor cells and whether this represents an important mechanism for effector cell binding to tumor cells by way of C receptors. (3)\Examine the role of the leukocyte C system in the neutrophil respiratory burst response to zymosan and bacteria. Determine whether either iC3b or specific carbohydrates contained in the microorganism cell walls trigger a respiratory burst and release of lysosomal enzymes through recognition by C receptor type three (CR3). (CS)